1. Field of the Invention
The present invention relates to an electronic equipment equipped with a position detector or detecting means for detecting the position of a moving body using an incremental type encoder.
2. Description of Related Art
Electronic equipment for measuring the position of a moving body and electronic equipment for exerting control in such a manner as to decide the position of a target position for a moving body is equipped with detection means for detecting the current position of the moving body. Typically, an absolute type encoder is used for obtaining absolute position information as means for detecting the position of movement of the moving body regardless of whether the movement is rotational or linear. However, in the case of an absolute type encoder problems arise such as an increase in cost that accompanies the signal processing circuitry being large and complex, and incremental type encoders are therefore widely used. The position of the moving body for carrying out rotational movement is directly detected using an incremental type encoder. Further, the position of a moving body moving in a linear manner is detected indirectly by detecting the position of a rotor of a motor driving the moving body using an incremental type encoder. An optical attenuator with a rocking moving body using an incremental type encoder. An optical attentuator with a rocking moving body and an optical linear filter with a reciprocally moving body, for example, may be taken as the electronic equipment equipped with position detection means for detecting the position of a moving body using an incremental type encoder.
As shown in FIG. 3, an incremental type encoder has a known readable member such as a disk-shaped encoder scale 31 formed from a large number of slits for detecting changes in the amount of rotation constituted by rows of inner and outer slits, i.e. inner slits 34a and outer slits 34b, and one absolute position slit 34z for detecting absolute position information. The position of the inner slit 34a and the outer slit 34b is decided in such a manner that phases of pulse signals generated based on an optical beam passing through the slits differ by ninety degrees.
The following is a detailed description with reference to FIG. 2 of the structure of an encoder fitted to an ultrasonic motor. In FIG. 2, the moving body 24 is a rotor supported in a freely rotatable manner at an axis 27 via a bearing 28. The axis 27 is installed so as to stand up at a motor substrate 29 and the disk-shaped oscillating body 23 is fixed to the same axis as the axis 27. A plurality of projections 26 are integrally formed at the upper surface of the disk-shaped oscillating body 23, while a piezoelectric material 22 is connected to the lower surface. A pair of electrode plates formed in a prescribed electrode pattern, i.e. an electrode 21a for rotation in a clockwise direction and an electrode 21b for rotation in an anti-clockwise direction are arranged at the lower surface of the piezoelectric material 22. The oscillating body 23 that also functions as a common electrode plate for the upper surface of the piezoelectric material 22 is electrically connected to a common electrode 21c. A spring 25 constituting pressing means pushes the rotor 24 in the direction of the oscillating body 23 and presses against upper end surfaces of the plurality of projections 26 at the lower surface of the rotor 24.
The disk-shaped encoder scale 31 shown in FIG. 3 is arranged so as to be attaching to the upper surface of the rotor 24 of the ultrasonic motor. A light emitting element 32 comprising 32a, 32b and 32c (not shown) and a light receiving element 33 comprising 33a, 33b and 33c (not shown) are positioned so as to respectively correspond to the inner slit 34a, the outer slit 34b and the absolute position slit 34z and are mounted to the motor substrate 29 in such a manner as to sandwich the disk-shaped encoder scale 31. The light beam from the light emitting element 32 is then received by the light receiving element 33 after passing through the inner slit 34a, the outer slit 34b and the absolute position slit 34z. 
As described above, the position of the inner slit 34a and the outer slit 34b is decided in such a manner that phases of pulse signals generated based on an optical beam passing through the slits differ by ninety degrees. The output pulse signal of the incremental type encoder therefore becomes as shown in FIG. 6. Phase A is a pulse signal generated based on a light beam passing through the inner slit 34a and phase B is a pulse signal generated based on a light beam passing through the outer slit 34b. FIG. 6A shows an output pulse signal for the case of rotation in a clockwise direction CW and FIG. 6B shows an output pulse signal for the case of rotation in an anti-clockwise direction CCW.
The signal used in position detection is a single four times multiplied signal obtained from the A phase signal and the B phase signal. As shown in FIG. 7, the four times multiplied signal is a pulse signal generated using the rising edge and the falling edge of the two output pulse signals for an A phase and B phase that differ in phase by ninety degrees. Resolution that is four times that of the A phase signal or the B phase signal can therefore be obtained by using this signal in position detection. A Z phase signal that provides absolute position information is a pulse signal with a pulse width that is approximately twice that of the A phase signal or the B phase signal.
An incremental type encoder is capable of detecting both forward and reverse rotation and can detect position with a high degree of resolution and is therefore widely utilized as a means of detecting the position of movement of moving bodies. In this case, the output pulse signal of the incremental type encoder is counted by a counter and the position of the moving body is detected based on this count value. Therefore, with an incremental type counter where the count value for the starting point position is set to zero, the count value for the case of rotation in a clockwise direction by an m pulse portion is +m, with the count value returning to zero in the case of rotation by an m pulse portion in an anti-clockwise direction. Similarly, the count value for the case of rotation in an anti-clockwise direction by an n pulse portion is −n, with the count value returning to zero in the case of rotation by an n pulse portion in a clockwise direction. In either case, if the start point is returned to, the count value is returned to zero. However, errors become included in the position information when an incremental type counter is subjected to noise. There is also a problem in that such errors remain during use. In other words, there is a problem that there is a shift in the starting point of the position detection means die to noise.
A first problem to be resolved is to ensure that with electronic equipment equipped with position detection means for detecting the position of a moving body using an incremental type encoder and an incremental type counter, a starting point of the position detection means is not subjected to the influence of noise, etc. A second problem to be resolved is to ensure that with electronic equipment equipped with position detection means for detecting the position of a moving body using an incremental type encoder and an incremental type counter, an increase in reliability and a decrease in power consumption are achieved.